Business-centric view:

Business Model – how a company creates value for itself while delivering products and services for customers.
Steve Blank

A Business Model describes the rationale of how an organisation creates, delivers, and captures value, in economic, social, cultural, or other contexts. Model Generation, Alexander Osterwalder.

A Business Model specifies how the firm creates value (and for whom), and how it captures value (and from whom).
Unlocking the Customer Value Chain, Thales, S. Teixeira.

Customer-centric view:

A Business Model consists of the value a business creates for me, what it charges me in exchange for that value, and what value it erodes for me.
Unlocking the Customer Value Chain, Thales, S. Teixeira.

System-centric view:

A Business Model is a sustainable system of value exchange between a business and its customers, where both have their needs met.